A plane emission device emitting lights by light or electric energy, which is used for a light source of various kinds of display units, a display device, a back light, or a lighting system, has many superior characteristics such as high-luminance, high-efficiency, low-profile and light-weight. Among the plane emission devices, an organic electroluminescence device (organic EL device), which makes organic materials emit lights by electric energy from an anode and a cathode, is energetically researched and developed for practical use, because it enables high-luminance plane emission with a low voltage about several Volt, and enables the light emission with arbitrary color tone by selection of luminescent materials.
The organic EL device is, as shown in FIG. 3A, configured of a transparent electrode layer 7 which is formed on a surface of a transparent substrate 1, a luminescent layer 8 which consists of an organic EL material and is formed on a surface of the transparent electrode layer 7, an opposing electrode 9 which is formed on a surface of the luminescent layer 8, and so on. Then, lights emitted in the luminescent layer 8 by applying a voltage between the transparent electrode layer 7 and the opposing electrode 9 transmits through and exits from the transparent electrode layer 7 and the transparent substrate 1.
Hereupon, in case of a lighting device consisting of a thin film like the organic EL device, exit angles of the lights emitted from the luminescent layer 8 which is thin film of a luminous body are defined by refraction index of the luminescent layer 8 and refraction indexes of media into which the emitted lights pass through, that is, the transparent electrode layer 7 and the transparent substrate 1 when the emitted lights exit. When the exit angle of the light is equal to or larger than the critical angle, the light is reflected as total reflection on a boundary face and it is confined in inside of the luminescent layer 8 and so on, and consequently, it is lost as guided wave. In calculation of the classic optics of Snell's law, when the refraction index of the luminescent layer 8 is assumed as “n”, takeout efficiency η that the emitted lights can exit to outside is approximated as η≈1/(2n2). For example, when the refraction index of the luminescent layer 8 is 1.7, η≈around 17%, and thus, 80% or more of the lights is lost as guided light in lateral direction of the organic EL device, and consequently, the takeout efficiency of lights from the organic light emitting device becomes much lower.
In addition, in the case of the organic EL device using a fluorescent material for the luminescent material, among exciters which are generated by recombination of electric charges injected from the electrodes 7 and 9, it is only singlet exciters that contribute the light emission, and generation probability thereof is ¼. In other words, in consideration of only this point, efficiency becomes much lower of around 5%. In contrast, recently, a luminescent material by which light emission can be obtained from phosphorescence due to triplet exciters is developed for a method to increase the luminous efficiency of the luminescent layer in itself, so that a possibility that can improve the quantum efficiency drastically is found.
However, even if the quantum efficiency is drastically improved as above, when the takeout efficiency of lights is lower, it causes to decrease the luminous efficiency of the organic EL device due to the fact. In other words, if the takeout efficiency is improved, the luminous efficiency may be improved drastically as synergistic result.
In case of the organic EL device which is formed by laminating the luminescent layers 8 via the transparent electrode layers 7 on the surface of the transparent substrate 1, the lost light to amount to about 80% as above is mainly caused by (1) the total reflection on the boundary face between the transparent substrate 1 and air, (2) total reflection on the boundary face between the transparent electrode layer 7 and the transparent substrate 1. In other words, when a light enters into a medium of lower refraction index from a medium of higher refraction index, the light having an incident angle equal to or larger than the critical angle is total-reflected on the boundary face in concept of the total reflection, and thus, it is confined therein. When calculating ratios of (1) the total reflection on the boundary face between the transparent substrate 1 and the air and (2) the total reflection on the boundary between the transparent electrode layer 7 and the transparent substrate 1 in consideration with solid angles respectively, the former becomes about 35% and the latter becomes about 45%.
Therefore, two means of (1) reforming of the boundary face between the transparent substrate 1 and the air and (2) reforming of the boundary face between the transparent electrode layer 7 and the transparent substrate 1 are considered as the means to improve the takeout efficiency, now. With respect to the former case, a method to improve the takeout efficiency of lights by providing a scattering layer of a lens structure on the surface of the transparent substrate 1 (see Japanese Patent No. 2931211, for example) and a method to form a diffraction grating (see Japanese Unexamined Patent Publication No. 10-321371, for example) are proposed.
With respect to the latter case, as shown in FIG. 3B, a light scattering layer 2 is formed between the transparent electrode layer 7 and the transparent substrate 1 so as to disperse lights by the light scattering layer 2. Thereby, the ratio of the total reflection occurred on the boundary face between the transparent electrode layer 7 and the transparent substrate 1 is reduced, and thus, the takeout efficiency of lights is increased. Although the light scattering department (SIC) 2 is formed by providing minute irregularity on a surface of the transparent substrate 1 facing the transparent electrode layer 7, or providing a coating resin layer containing particles on a surface of the transparent substrate 1 facing the transparent electrode layer 7, at nay event, the surface becomes irregular. Therefore, as shown in FIG. 3B, an alleviation layer 31 is formed on a surface of the light scattering layer 2 for smoothing the irregularity, and the transparent electrode layer 7 having a thin thickness is formed on the smoothed surface of the alleviation layer 31 (see Japanese Unexamined Patent Publication No. 2003-216062 or 2006-286616, for example).
However, when the alleviation layer 31 is formed between the light scattering layer 2 and the transparent electrode layer 7 as above, the lights are total-reflected on a boundary face of the alleviation layer 31 and the transparent electrode layer 7, and thus, the takeout efficiency of lights is reduced due to the total reflection, therefore, it is desired to further improve the takeout efficiency.